Triacetylcellulose (TAC, also called cellulose triacetate) film has traditionally been used by the photographic industry due to its unique physical properties and flame retardance. TAC film is also the preferred polymer film for use as a cover sheet for the polarizers used in liquid crystal displays. It is the preferred material for this use because of its extremely low in-plane birefringence. Its out of plane birefringence is also small (but not zero), and is useful in providing some optical compensation to the LCD.
Intrinsic birefringence describes the fundamental orientation of a material at a molecular level. It is directly related to the molecular structure (bond angles, rotational freedom, presence of aromatic groups, etc.) of the material. The intrinsic birefringence is not affected by process conditions (temperature, stresses, pressures) used to make a macroscopic object.
Crystalline and liquid crystalline materials have the convenient property that their intrinsic birefringence manifests itself almost perfectly when they are assembled into a macroscopic article. Layers of crystalline and liquid crystalline molecules often can be manufactured such that all the molecules in the article are in registry with each other and thus preserve their fundamental orientation. The same is not true when making layers of an amorphous polymeric material. Their intrinsic birefringence can be highly modified by the manufacturing process. Thus, the measured birefringence of an actual article will be a resultant of its intrinsic birefringence and the manufacturing process. Because we are dealing with such amorphous polymeric materials, the following definitions refer to this measured birefringence and not intrinsic birefringence.
“In-plane birefringence” means the difference between nx and ny, where x and y lie in the plane of the layer. nx will be defined as being parallel to the casting direction of the polymer, and ny being perpendicular to the casting direction of the polymer film. The sign convention used will be nx−ny.
“Out-of-plane birefringence” means the difference between nz and the average of nx and ny, where x and y lie in the plane of the layer and z lies in the plane normal to the layer. The sign convention used will be: nz−[(nx+ny)/2]. TAC typically has a negative out of plane birefringence as its nz is smaller than its nx and ny.
“In-plane retardation (Re)” means the product of in-plane birefringence and layer thickness (t). Thus Re=t(nx−ny)
“Out-of-plane retardation (Rth)” means the product of out-of-plane birefringence and layer thickness (t). Thus Rth=t(nz−[(nx+ny)/2]).
Synthetic polymer films (such as polycarbonate or polysulfone) are often used to enhance the minimal optical compensation that TAC provides. These synthetic polymers films are attached to the rest of the display by adhesive lamination.
Generally in the field of optical materials, the synthetic polymer film is used as an optically anisotropic film (having a high retardation value), while a TAC film is used as an optical isotropic film (having a low retardation value).
Japanese Published Patent Application JP1999-95208 describes a liquid crystal display having an optical compensator (having high retardation) prepared by uniaxial stretching of a high polymer film. Such polymers include polyesters, polycarbonate or polysulfone. This stretching step is essential to obtain the desired optical properties. This stretching affects both in- and out-of-plane retardation simultaneously. These two orthogonal retardations cannot be independently controlled by this method. Also, producing uniform optical compensators by this method is described as being difficult. This application also describes a compensator where the inventor uses an exfoliated inorganic clay material in a polymeric binder coated on top of a TAC support. The exfoliated inorganic clay material in this layer is the optically active material, not the polymeric binder.
World patent WO 01/31394 A2 discusses the use of the color filter array layer as a source of additional out-of-plane retardation for a liquid crystal display. The color filter array is located within the constraints of the liquid crystal cell. The use of an aromatic polyimide binder rather than a polyacrylate binder for the color filter array dyes provides enhanced retardation. The overall retardation is achieved with the combination of the color filter array retarder plus optional additional out-of-plane retardation from the TAC used as a supporting member for the polarizers.
The proposal to select the binder for the color filter array with retardation in mind has an advantage versus polarizer-based retarders that are laminated to the liquid crystal cell: mechanical stresses to the display induced by room condition changes or perhaps direct shock can cause polarizer-based retarders to move relative to the liquid crystal cell. Retarders coated directly on the glass substrate are more rigidly held in registry with the cell, and thus do not suffer this problem. However the requirement that this color filter array be also a retarder means that this layer must serve two purposes: color filtering and adding retardation. This limits the potential thickness to be considered for this layer. This layer must also be pixilated, and this adds additional complications. Finally it is taught on the internal surface of the constraint only, where the color filter array is located.
It is a problem to be solved to provide a liquid crystal cell that is readily manufactured and that readily provides the required degree of in-plane and out-of-plane compensation while reducing the problems associated with a laminated compensator.